1. Field of the Invention
The present invention relates to the field of spectral analysis and, more specifically, toward the identification of the type and quantity of different chemicals in solid (e.g., soft) and liquid samples by transmitting light of varying wavelengths through the sample so as to provide wavelength information and to provide measured amounts of light absorbed by any given sample.
2. Discussion of the Related Art
A molecular spectrometer is an instrument wherein a solid, liquid, or gaseous sample is illuminated, often with non-visible light such as light in the infrared region of the spectrum. The light transmitted through the sample is then captured and analyzed to reveal information about the characteristics of the sample. As an example, a sample may be illuminated with infrared light having a known intensity across a range of wavelengths, and the light transmitted by the sample can then be captured for comparison to the light source. Review of the captured spectra can then illustrate the wavelengths at which the illuminating light was absorbed by the sample. The spectrum, and in particular the locations and amplitudes of the peaks therein, can be compared to libraries of previously obtained reference spectra to obtain information about the sample, such as its composition and characteristics. In essence, the spectrum serves as a “fingerprint” for the sample and for the substances therein, and by matching the fingerprint to one or more known fingerprints, the identity and the quantity of the sample might be determined.
However, there are numerous occasions when the data collected using such above described methods is useless because the transmitted light is substantially absorbed by too large of a path length, or the light is nearly totally transmitted by too small of path length. Either one of these situations can be problematic. With respect to a large absorbance (e.g., due to a large path length), uncertainties based on noise (i.e., signal-to-noise) become problematic as the spectral signal of the sample is lost due to the light being too weak to be reliably detected within a larger signal. However, if the absorbance signal is too small (e.g., due to a small path length), reliable detection is still a problem also because of a lack of absorbance signal strength even though overall light level is high. By varying the path length through the sample, both of these problems can be minimized. Other uncertainties in both situations also can include natural variations in the light intensity caused by dirt, dust in the light beam, temperature, vibration variations affecting the measurement means, and/or finally statistical variation the light source and detection system.
Background information on an apparatus and methodology that provides for measuring optimized absorbance properties of a liquid droplet can be found in U.S. Pat. No. 7,365,852, to Schleifer, issued Apr. 29, 2008, entitled; “Methods and Systems for Selecting Pathlength in Absorbance Measurements,” including the following: “[m]ethods and sub-systems for substantially optimizing the absorbance measurement in optical instruments are provided. A method comprises forming a liquid sample into a droplet extending between opposing surfaces, passing a light beam through the sample, and varying the distance between the two opposing surfaces until a distance substantially corresponding to a optimum absorbance is obtained.”
Background information on an apparatus and methodology that provides for measuring transmission properties of liquids and solids can be found in U.S. Pat. No. 7,582,869, to Sting et al., issued Sep. 1, 2009, entitled; “System and Method for Optical Analysis,” including the following: “[a]n optical analysis system utilizing transmission spectroscopy for analyzing liquids and solids includes a source of optical energy, a sample, a movable optical energy transmission window, a fixed optical energy transmission window, and a detection system. The fixed transmission window remains fixed relative to the source of optical energy. The sample is selectively positioned between the movable and fixed optical energy transmission windows for analyzing the sample. The optical energy is transmitted through one of the windows, the sample, and the other window to obtain encoded optical energy as a result of transmitting the optical energy through the sample. A detection system receives the encoded optical energy for analysis. The movable optical energy transmission window is selectively movable relative to the fixed optical energy transmission window to repeatedly and precisely align and make readily accessible both windows and the sample.”
Background information on an apparatus and methodology that provides for measuring transmission properties of compressed samples can be found in EP 1, 792, 653, to Juhl, issued Jun. 6, 2006, entitled; “Apparatus and method for spectrophotometric analysis,” including the following: “[a]n apparatus for spectrophotometric analysis comprises a sample reception surface, which is arranged to receive a sample to be analysed, and a sample contacting surface, which is moveable in relation to the sample reception surface such that it may be brought to a first position, where the surfaces are sufficiently far apart to allow the sample to be placed on the sample reception surface, and a second position, where the sample contacting surface makes contact with the sample and compresses the sample. The apparatus further comprises a sample thickness controller, which is arranged to control the distance between the sample reception surface and the sample contacting surface in the second position of the sample contacting surface, such that a sample thickness between the surfaces may be shifted for obtaining at least two measurements of the sample at different optical path lengths through the sample.”